1. Technical Field
The present invention relates to a color conversion device, an image output device, and a color conversion method.
2. Related Art
A color range (color reproducibility range) of an image output device in related art such as an image display apparatus including display device and projector and a printing device including color printer is not sufficiently wide for representing color of an existing object due to the limitation that the color range is determined by the characteristics of fluorescent material contained in CRT (cathode ray tube), or for other reasons. On the other hand, with development of color reproducibility technology, a new type of image output device having expanded color range has been available on the market, and color spaces having expanded color range have been already used for processing still images and dynamic images.
Thus, for meeting the demand for color space standards (color signal standards) capable of dealing with wider color range, various wide color range standards such as xvYCC (IEC61966-2-4) have been established. Particularly the xvYCC color space has much wider color range than those of sRGB color space capable of representing standard color range space of the image display apparatus and AdobeRGB color space and NTSC (national television standards committee) color space having wider color range than that of the sRGB color space. Thus, the xvYCC color space can theoretically represent so-called imaginary color and colors not used in practical representation.
Since image signals for representing an image are specified by various types of color space standards, there is a possibility that the color range of image signals on the image input device side is different from the color range of image signals on the image output device side. In this case, color conversion process is performed for the image signals generated from the image input device side so that color reproducibility can be secured according to the color space standards on the image output device side.
JP-A-2007-251709 discloses this type of color conversion device for performing the color conversion process. According to the technology disclosed in this reference, for conversion from YCC signal in luminance color difference system into RGB signal in display signal system, YCC signal is converted into RGB signal by matrix calculation before execution of color conversion with reference to RGB color control LUT (look up table) so as to increase utilization efficiency of RGB color control LUT.
Also, JP-A-2007-288590 discloses a technology which converts input color signals in color space dependent on input device into signals in color space independent of the input device, converts the color signals as converted signals in the color space independent of the input device into color signals in virtual input color space containing color reproducibility range of a general input device, and converts the color signals as converted signals in the virtual input color space into color signals in color space dependent on an output device by using CLUT.
According to the technology disclosed in JP-A-2007-251709, color conversion is performed using the RGB color control LUT. Thus, accuracy of the color conversion process decreases when the number of lattice points of LUT is small. In this case, the number of lattice points of LUT needs to be increased for accurate color conversion, which raises cost.
According to the technology disclosed in JP-A-2007-288590, the maximum saturation point in the virtual input color space is so determined as to coincide with the lattice point of CLUT. However, the maximum saturation point in the input color space is not so determined as to coincide with the lattice point of CLUT during conversion from input device dependent color space into input device independent color space and conversion from input device independent color space into virtual input color space. Thus, error is caused by interpolation calculation for the maximum saturation point of input color space.